Fabrication of semiconductor devices using anti-reflective coatings

ABSTRACT

Techniques are disclosed for fabricating a device using a photolithographic process. The method includes providing a first anti-reflective coating over a surface of a substrate. A layer which is transparent to a wavelength of light used during the photolithographic process is provided over the first anti-reflective coating, and a photosensitive material is provided above the transparent layer. The photosensitive material is exposed to a source of radiation including the wavelength of light. Preferably, the first anti-reflective coating extends beneath substantially the entire transparent layer. The complex refractive index of the first anti-reflective coating can be selected to maximize the absorption at the first anti-reflective coating to reduce nothing of the photosensitive material.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 10/684,431, filed Oct. 15, 2003 now U.S. Pat. No. 7,067,894,which is a continuation of U.S. patent application Ser. No. 09/252,448,now U.S. Pat. No. 6,713,234, filed Feb. 18, 1999, the disclosures ofwhich are herewith incorporated by reference in their entirety.

BACKGROUND

The present invention relates generally to the manufacture ofsemiconductor devices and, in particular, to methods for fabricatingsuch devices using anti-reflective layers, as well as devices includinganti-reflective coatings.

The fabrication of integrated circuits requires the precise positioningof a number of regions in a semi-conductor wafer, followed by one ormore interconnection patterns. The regions include a variety of implantsand diffusions, cuts for gates and metallizations, and windows inprotective cover layers through which connections can be made to bondingpads. A sequence of steps is required for each such region.

Photolithographic techniques, for example, can be used in theperformance of some or all of the foregoing operations. Typically, forexample, the surface of a wafer to be processed is pre-coated with aphotoresist. The photoresist then is exposed to a light source with asuitably patterned mask positioned over the wafer. The exposed resistpattern is used, for example, to open windows in a protective underlyinglayer to define semiconductor regions or to delineate an interconnectionpattern.

To improve the degree of integration and to obtain high density devices,performing photolithographic operations at shorter wavelengths isdesirable. Currently, i-line techniques with a wavelength of about 365nanometers (nm), KrF excimer laser techniques with a wavelength of about248 nm, and KnF excimer laser techniques with a wavelength of about 193nm are used. However, at those wavelengths, optical reflections at theinterfaces of previously-formed layers on the semiconductor wafer cancause notching of the photoresist.

FIG. 1 illustrates the general nature of the problem. A semiconductorwafer 10 includes one or more previously-formed layers 12 covered by athick layer of boro-phospho-silicate glass (BPSG) 14. The BPSG layer 14serves as a protective layer for the underlying layers 12 and alsoprovides a more planar surface. A photoresist film 16 is coated over theBPSG layer 14, and a mask 18 is positioned over the photoresist prior toexposure of the photoresist to an appropriate source of radiation 20.The mask 18 can be used to define, for example, contact holes for one ofthe previously-formed layers 12.

Ideally, when the photoresist film 16 is exposed to the radiation 20,the mask 18 precisely defines the dimensions of the exposed regions ofthe photoresist film. However, the BPSG layer 14 is transparent to thewavelengths typically used in photolithography, including 248 nm and 365nm. Thus, a significant amount of the radiation 20 that passes throughthe mask 18 travels through the BPSG layer 14 and is reflected at theinterface between the BPSG layer and one or more of thepreviously-formed underlying layers 12. Some of the reflected radiation(indicated by arrow 22) contributes to exposure of the photoresist film16.

In some situations, a dielectric anti-reflective coating is providedabove the BPSG layer to reduce reflections from the underlying layers.However, if the structures in the previously-formed underlying layers 12have varying dimensions or varying shapes and the level of reflectedlight is relatively high, the reflected light 22 will expose thephotoresist film 16 non-uniformly leading to the formation of notching.

SUMMARY

In general, techniques are disclosed for fabricating a device using aphotolithographic process. The techniques are particularly advantageousfor transferring an optical pattern by photolithography to one or morelayers which are transparent to the wavelength(s) at which thephoto-lithography is performed.

According to one aspect, a method includes providing a firstanti-reflective coating over a surface of a substrate. As used herein,the term “substrate” refers to one or more semiconductor layers orstructures which may include active or operable portions ofsemiconductor devices. Various films or other materials may be presenton the semiconductor layers or structures. A layer which is transparentto a wavelength of light used during the photolithographic process isprovided over the first anti-reflective coating, and a photosensitivematerial is provided above the transparent layer. The photosensitivematerial is exposed to a source of radiation including the wavelength oflight. Preferably, the first anti-reflective coating extends beneathsubstantially the entire transparent layer.

According to another aspect, a semiconductor device includes a layerthat is transparent to light having a wavelength, for example, ofapproximately 193 nm, 248 nm or 365 nm. A first anti-reflective coatingextends substantially entirely beneath the transparent layer.

One advantage of providing an anti-reflective coating beneath thetransparent layer is that the anti-reflective coating can help reducenotching of the photosensitive material that may occur during thephotolithographic process.

In general, the complex refractive index of the first anti-reflectivecoating can be selected to maximize (or increase) the absorption at thefirst anti-reflective coating to minimize (or reduce) the amount oflight transmitted through the first anti-reflective coating andreflected back from the underlying structures. Therefore, the effects ofthe non-uniform structures in the layers below the first anti-reflectivecoating can be reduced or eliminated. That, in turn reduces the amountof light that is reflected back toward the photosensitive material and,therefore, further reduces notching.

Various implementations include one or more of the following features.The transparent layer can include a material such as BPSG, PSG and TEOS.Other materials, including various oxides and nitrides, also can be usedas the transparent layer. Depending on the properties of thephotosensitive material, it can be exposed to radiation at variouswavelengths including approximately 193 nm, 248 nm or 365 nm. Portionsof the photosensitive material selectively can be exposed to theradiation.

The first anti-reflective coating can include various materials,including a material comprising silicon and nitrogen; silicon andoxygen; or silicon, oxygen and nitrogen. Other materials, such asorganic polymers, also can be used as the first anti-reflective coating.

According to another aspect, in addition to the first anti-reflectivecoating, a second anti-reflective coating can be provided above thetransparent layer. The photosensitive material then can be provided overthe second anti-reflective coating. Providing the second anti-reflectivecoating between the photosensitive material and the transparent layercan further help reduce the effects of light that is reflected from theinterface of the first anti-reflective coating and the transparentlayer.

Other features and advantages will be readily apparent from thefollowing description, the accompanying drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-section of a semiconductor wafer duringperformance of a photolithographic process.

FIG. 2 illustrates a cross-section of an exemplary, partially-fabricatedsemiconductor device.

FIG. 3 illustrates a cross-section of the device of FIG. 2 duringperformance of a photolithographic process according to the invention.

FIG. 4 illustrates a cross-section of the device of FIG. 3 followingformation of a contact hole.

FIG. 5 illustrates a cross-section of a device having multiple layers towhich a photolithographic pattern is to be transferred and which aretransparent to the light used during the photolithographic process.

FIG. 6 illustrates a cross-section of a device during performance of aphotolithographic process according to another embodiment of theinvention.

DETAILED DESCRIPTION

As explained in greater detail below, a technique is described fortransferring an optical pattern by photo-lithography to one or morelayers which are transparent to the wavelength(s) at which thephotolithography is performed. The technique is described with respectto the formation of a contact hole through a BPSG layer. However, thetechnique is generally applicable to the formation of various featuresin integrated devices involving the transfer of a pattern byphotolithography to other transparent layers as well. Such layersinclude, for example, phospo silicate glass (PSG),tetra-ethyl-ortho-silicate (TEOS), undoped oxides, among others.

As shown in FIG. 2, an exemplary partially-fabricated semiconductordevice includes an active region 30 formed on a semiconductor wafer 32and surrounded by an isolation oxide film 34. A gate electrode 36 isformed in the active region 30 with a gate oxide film 38 disposedbetween the electrode and the active region. The gate electrode 36includes a first polycrystalline silicon (poly-Si) film 40 and a firstrefractory metal silicide film 42. The gate electrode 36 has its topportion covered with a silicon oxide film 44 and its sidewalls coveredwith an insulation film 46.

Impurity diffusion layers 48 form source and drain regions at the uppersurface of the wafer 32 at either side of the gate electrode to providea metal-oxide-semiconductor (MOS) field effect transistor structure. Asilicon oxide layer 56 is provided over the surface of the substrate.

A first interconnection layer 50, including a second poly-Si layer 52and a second refractory metal silicide layer 54, is formed on one of theimpurity diffusion layers 48. As shown in FIG. 2, a silicon oxide film58 is formed over the entire upper surface of the substrate.

Techniques for forming the foregoing layers are well-known and,therefore, are not described in further detail.

Prior to forming a protective BPSG layer, a first dielectricanti-reflective coating (ARC) 60 is formed over substantially the entireupper surface of the substrate (FIG. 3). In one implementation, thefirst dielectric ARC 60 has a thickness in the range of approximately200-500 angstroms (Å), although in general, the thickness will varydepending on the particular application and can be greater than 500 Å orless than 200 Å.

Next, a BPSG layer 62 is deposited on the first dielectric ARC 60, forexample, by vapor deposition. The BPSG layer 62 can be heated at atemperature of approximately 850 celsius (° C.) to form an interlayerinsulation film having a relatively flat surface. The thickness of theBPSG layer 62 is typically much greater than the thickness of the firstdielectric ARC 60 and may be as great as 20,000 Å. Preferably, the firstARC 60 extends substantially beneath the entire BPSG layer 62.

A second dielectric ARC 64 is formed over substantially the entire uppersurface of the BPSG layer 62. The second dielectric ARC 64 also caninclude, for example, a silicon oxide (Si_(x)O_(y):H) film, a siliconnitride film (Si_(x)N_(y):H) or a silicon oxy-nitride(Si_(x)O_(y)N_(z):H) film. Other materials also can used for the seconddielectric ARC 64. In one implementation, the second dielectric ARC 64has a thickness in the range of approximately several hundred angstroms(Å), although in general, the thickness will vary depending on theparticular application and can be greater. or less.

A photosensitive film, such as a photoresist 66, is deposited over thesecond dielectric ARC 64, and a mask 68 is positioned over thephotoresist prior to exposure of the photoresist to an appropriatesource of radiation 70. Depending on the properties of the photoresist,the radiation which exposes the photoresist can include one or moredifferent wavelengths. Exemplary sources of radiation include those usedin i-line techniques with a wavelength of about 365 nanometers (nm), KrFexcimer laser techniques with a wavelength of about 248 nm, and KnFexcimer laser techniques with a wavelength of about 193 nm. In theillustrated example of FIG. 3, the mask 68 is used to define contactholes for one of the previously-formed layers.

One advantage of providing the dielectric ARC 60 immediately below theBPSG layer 62 is that the ARC 60 can reduce the amount of notching thatmay result during the photolithographic process for defining the contactholes. In general, the complex refractive index of the first ARC 60 canbe selected to maximize the absorption at the first anti-reflectivecoating to minimize amount of transmitted light through the firstanti-reflective coating back from the underlying structures. Therefore,the effects of the non-uniform structures in the layers below the firstARC can be reduced or eliminated. That, in turn reduces the amount oflight that can be reflected back toward the photo-sensitive materialand, therefore, further reduces notching.

The complex refractive index of a material includes a real part n, knownas the refractive index and defining the velocity of light in thematerial, and an imaginary part k which corresponds to the material'slight absorption coefficient. In the discussion that follows, the realand imaginary parts (n, k) of the complex refractive index for thevarious layers will be referred to as follows:

Layer n k Underlayer n₁ k₁ First ARC (60) n₂ k₂ BPSG (62) n₃ k₃ SecondARC (64) n₄ k₄

The reflectance and transmittance at the interface of the BPSG layer 62and the first dielectric ARC 60 are determined by n₂, k₂, n₃ and k₃. Toincrease the absorption in the first dielectric ARC 60, k₂ should berelatively high, for example, at least as high as 1.0 and preferably ashigh as 1.5.

The first dielectric ARC 60 can include, for example, a silicon oxide(Si_(x)O_(y):H) film, a silicon nitride (Si_(x)N_(z):H) film or asilicon oxy-nitride (Si_(x)O_(y)N_(z):H) film. Other materials,including titanium nitride and organic compounds such as polymers, alsocan used for the first dielectric ARC 60. In one implementation, aSi_(x)O_(y)N_(z):H layer can be formed as the ARC 60 using aplasma-enhanced vapor deposition technique. A film including a mixtureof silicon (Si), oxygen (O), nitrogen (N) and hydrogen (H) atoms can beformed by exciting a plasma in a gas mixture of silane and nitrogenoxide (N₂O) diluted by helium (He). For example, to obtain aSi_(x)O_(y)N_(z):H layer with a value of n equal to about 2.13 and avalue of k equal to about 1.23 for light having a wavelength of 248 nm,the flow rates of silane, N₂O and He can be approximately 80 sccm, 80sccm and 2200 sccm, respectively. The Si_(x)O_(y)N_(z):H layer is formedat a temperature of about 400° C. and a pressure of about 5.6 Torr. TheRF power can be set to approximately 105 watts.

If the topography of the underlying structure below the first ARC 60 isnot substantially planar, then light reflected from the interface of thefirst ARC and the BPSG layer 62 will tend to scatter in variousdirections non-uniformly. Providing the second ARC 64 between thephotoresist layer 66 and the BPSG layer 62 can help reduce the effectsthat any such non-uniform light has on the photoresist.

The second ARC 64 also can include, for example, a Si_(x)O_(y):H film, aSi_(x)N_(z):H film or a Si_(x)O_(y)N_(z):H film. Other materials,including titanium nitride and organic compounds such as polymers, alsocan be used for the second dielectric ARC 64. In general, the values ofn₄ and k₄ for the second ARC 64 should be selected to minimize or reducethe reflectivity at the interface between the photoresist layer 66 andthe second ARC 64. Selecting a suitable material for the second ARC 64can be determined using known simulated techniques. Generally, however,selection of the material for the second ARC 64 will depend on the BPSGlayer 62 as well as on the first ARC 60. The layers below the first ARC60 can be ignored due to the high absorption of the first ARC 60.

Once the photolithographic process is performed and the photoresist 66is developed, the ARC 64, the BPSG layer 62, the ARC 60, the siliconoxide layers 56, 58 and the gate oxide film 38 are etched successivelyto form a contact hole 72 (FIG. 4). The remaining resist 66 then can beremoved, and fabrication of the device, including the formation ofmetallization contacts, can be completed using conventional techniques.Upon completion of the device, the first anti-reflective coating 60 willextend beneath substantially the entire transparent layer 62.

In some cases, the layer which is transparent to the wavelength(s) oflight may include multiple vertically stacked layers 62A, 62B (FIG. 5)rather than a single layer, where each of the multiple layers 62A, 62Bis transparent to the wavelength(s) of light used during thephotolithographic process.

In situations where the topography of the underlying structures belowthe first anti-reflective coating is substantially planar, the secondanti-reflective coating need not be provided. As shown in FIG. 6, asemiconductor wafer 80, which may include one or more previously-formedlayers or regions, has a substantially planar topography. Ananti-reflective coating 82 is provided over the wafer 80. Thecomposition of the anti-reflective coating 82 can be similar to thosediscussed above with respect to the anti-reflective coating 60. Ingeneral, as previously discussed, the complex refractive index of theanti-reflective coating 82 should be selected to maximize the absorptionat the anti-reflective coating to minimize the amount of transmittedlight through the anti-reflective coating back from the underlyingstructures formed on the wafer 80. A BPSG or other protective layer 84is provided over the anti-reflective coating 82. A photo-lithographicpattern then is formed by providing a photosensitive film 86, such asphotoresist, over the protective layer 84 and exposing the photoresistto an appropriate source of radiation 90 with a mask 88 in place overthe photoresist. Although the protective layer 84 is transparent to theradiation 90, any light reflected from the interface of the protectivelayer and the anti-reflective coating 82 will tend to be reflectedsubstantially uniformly. Therefore, notching of the photoresist pattern86 can be reduced or eliminated.

Although the foregoing techniques have been described with respect to aprotective layer in a specific semiconductor device, an anti-reflectivecoating can be provided immediately below any layer to which aphoto-lithographic pattern is to be transferred and which is transparentto the wavelength of light used during the photolithographic process.

Other implementations are within the scope of the following claims.

1. A semiconductor device comprising: a gate electrode formed over asemiconductor substrate; first impurity region formed on one side ofsaid gate electrode; a second impurity region formed on the oppositeside of said gate electrode; an interconnection layer formed over saidfirst impurity region; a first dielectric anti-reflective coating layerformed of a material selected from the group consisting of siliconoxides, silicon nitrides, and silicon oxy-nitrides formed oversubstantially the entire upper surface of said semiconductor substrate;an interlayer insulation film formed over said first dielectricanti-reflective coating layer, wherein a contact hole is providedthrough said first anti-reflective coating layer and said interlayerinsulation film exposing said second impurity region; and a seconddielectric anti-reflective coating layer formed substantially on theentire upper surface of said interlayer insulation film.
 2. Thesemiconductor device of claim 1, wherein a contact hole is providedthrough said first and second anti-reflective coating layers and saidinterlayer insulation film exposing said second impurity region.
 3. Thesemiconductor device of claim 1, wherein said interlayer insulation filmcomprises a material that is transparent to light selected from thegroup consisting of BPSG, PSG, and TEOS.
 4. The semiconductor device ofclaim 1, wherein said first dielectric anti-reflective coating layer isapproximately from 200 Å to about 500 Å thick.
 5. The semiconductordevice of claim 1, wherein said first dielectric anti-reflective coatinglayer has a complex refractive index with an imaginary part k thatcorresponds to the first dielectric anti-reflective coating layer'slight absorption coefficient, wherein said k is in the range fromapproximately 1.0 to approximately 1.5.